Objective : Increase in reactive oxygen metabolites (ROM) and free radicals is an important cause of cell injury. In this study, we investigated whether determination of ROM in pleural fluids of patients with malignant and non-malignant pleural effusions can be used as a tumor marker indicating malignant effusions in the differential diagnosis. Methods :Sixty subjects with exudative pleural effusion and 25 healthy individuals as the control group were included in the study. Of the subjects with pleural effusion, 50 were malignant and 50 were non-malignant. ROM was studied in the pleural fluids and sera of the subjects with pleural effusion and in the sera of those in the control group. The ROM values of smokers and non-smokers were compared in each group. The Students t-test and the Mann-Whitney U test were used in order to detect differences between groups for descriptive statistics in terms of pointed features. The statistical significance level was set at 5 in computations, and the computations were made using the SPSS (ver.13) statistical package program. Results : It was determined that the difference between the ROM values of subjects with malignant and non-malign pleural effusions and the sera of the control group was significant in the malignant group compared to both groups (P = 0.0001), and the sera ROM values of patients with non-malignant pleural effusion were significant compared to the control group (P = 0.0001), and the ROM values of smokers were significant compared to non-smokers in each of the three groups (P = 0.0001). Conclusion : These findings indicate that sera ROM levels are increased considerably in patients with exudative effusions compared to that of the control group. This condition can be instructive in terms of serum ROM value being suggestive of exudative effusion in patients with effusions. Furthermore, the detection of pleural ROM values being significantly higher in subjects with malignant pleural effusions compared to non-malignant subjects suggests that ROM can be used as a tumor marker in the differential diagnosis of pleural effusions of unknown origin.